Phenomenological Studies Of Active-Sterile Neutrino Oscillations

In this thesis we focus on the phenomenology of neutrino oscillation, assuming the existence of sterile neutrinos. The thesis is organized as follows:Chap.1, Introduction. In this chapter, we shall give an overview of the history of neutrino physics, outline some fundamental knowledge of neutrinos that we have accumulated until nowadays, and summarize the research frontier by stating five groups of open questions on neutrinos.Chap.2, Neutrino Oscillation Theory. In this chapter, we first give a brief summary to the standard model of electroweak interaction including introductions of the construction of the Standard Model Lagrangian, and the Higgs mechanism which endows gauge bosons in a gauge theory with mass through absorption of Nambu-Goldstone bosons arising in spontaneous symmetry breaking. We then introduce the description of a massive neutrino in the cases of Dirac/Majorana and the most general Dirac-Majorana mass. After that, we introduce the PMNS matrix which describe the neutrino mixing, and the parameterization of the mixing matrix for arbitrary3+Ns neutrinos where Ns stands for the number of sterile neutrino. Finally, we derive the expressions of neutrino oscillation probabilities and the famous MSW equation which formulize neutrino oscillations in matter.Chap.3, Oscillation Experiments and SBL anomaly. In this chapter, we first introduce different types of neutrino experiments。We shall introduce carefully the SBL neutrino experiments and anomalies from their recent results. A neutrino oscillation explanation of these anomalies implies the existence of at least one extra mass-squared Difference ΔmS2BL such that ΔmS2OL<<ΔmA2TM<<AmS2BL and a small mixing of the three active neutrinos with extra light sterile neutrino states. In this case, all the analytical calculations and data analyses in the standard3-neutrino scheme should be revisited and extended in a more general scheme. Finally, we will present the results of global analyses of short-baseline neutrino oscillation data in3+1,3+2and3+1+1neutrino mixing schemes. We show that the data do not allow us to abandon the simplest3+1scheme in favor of the more complex3+2and3+1+1schemes. We present the allowed region in the3+1parameter space, which is located at Δm412between0.82and2.19eV2at3σ. The case of no oscillations is disfavored by about6σ, which decreases dramatically to about2a if the LSND data are not considered. Hence, new high-precision experiments are needed to check the LSND signal.Chap.4, Active-Sterile Solar Neutrino Oscillations and CP-Violating Phases. In this chapter, we extend the discussion of the solar neutrino oscillations in a general scheme of3+Ns mixing, without any constraint on the mixing between the three active and the Ns sterile neutrinos, assuming only a realistic hierarchy of neutrino mass-squared differences. We find that effects of CP-violating phases take place in the oscillation probabilities. A generalized Parke formula describing the neutrino oscillation probabilities inside the Sun is calculated. The validity of the analytical calculation and the probability variation due to the unknown CP-violating phases are illustrated with a numerical calculation of the evolution equation in the case of3+1neutrino mixing.Chap.5, Day-Night Asymmetries in Active-Sterile Solar Neutrino Oscil-lations. In this chapter, we discuss the day-night asymmetries in active-sterile solar neutrino oscillations in the framework mentioned in Chap.4. Analytical expressions of the probability of neutrino flavor transitions in the Earth in the perturbative approximation and in the slab approximation are presented and the effects of active-sterile mixing and of the CP-violating phases are discussed. The accuracy of the analytical approximations and the properties of the day-night asymmetries are illustrated numerically in the3+1neutrino mixing framework.Chap.6, Conclusion and Prospect. In this chapter, we shall give a short review and summary of the previous six chapters and try to describe the prospects of sterile neutrino phenomenology.